1. Field of the Invention
This invention relates generally to atomic absorption spectroscopy and more particularly to the flameless atomization of samples preparatory to analysis by means of atomic absorption spectroscopy.
2. Description of the Prior Art
While the atomization of samples for atomic absorption spectroscopy initially was accomplished by spraying a finely nebulized sample into a flame, at the present time, a substantial proportion of AA spectroscopic analysis employs what is referred to as flameless atomization in apparatus frequently referred to as a graphite furnace. Such a furnace most commonly takes the form of a single graphite tube mounted between two annular electrodes. A sample is inserted at or near the midpoint of the tube's length and the tube is heated by an electric current passed between the electrodes.
The current supply and, therefore, the degree of heating takes place in accordance with a specific program. Initially, a relatively small current is passed through the tube raising the temperature to the value required for drying of the sample, i.e., to evaporate a solvent, for example. Thereafter, the temperature of the tube is raised to the point required for thermal decomposition of the sample molecule, known as "ashing". Finally, a very high current is supplied raising the temperature of the tube to the point where atomization takes place so that the elements contained in the sample are present in an atomic state, forming an atomic cloud within the graphite tube.
A beam of radiation from a source of known spectral characteristics is passed axially through the tube to a detector. The source, customarily a hollow cathode lamp, is selected to produce radiation of a relatively narrow band of wavelengths corresponding to the resonance lines of the element sought to be detected. This radiation is absorbed by the atomic cloud in proportion to the concentration of the element in question, the degree of absorption being measured by the detector. In order to prevent combustion of the graphite tube at the high operating temperature involved, it is enveloped in a flowing stream of protective gas. For additional information regarding conventional graphite furnace-type sample cells, reference may be had to U.S. Pat. Nos. 3,778,156 and 3,788,752.
In a graphite furnace of the type described, the rate at which samples can be processed is limited to the thermal lag, i.e., the length of time required for the tube to heat from drying to ashing or ashing to atomization temperature after the heating current is increased. This delay limits the number of analyses which can be carried out in a given period of time.
Another drawback of prior apparatus is the fact that a reproducible measurement of signal peaks requires exact observation of the temperature and flow conditions extant during the analysis. The reason for this is that there is some absorption of the radiation beam and a concomitant output signal from the detector, if the atomic cloud is formed in the graphite tube while it is heating up to atomizing temperature. In addition, the atomic cloud is partially dispersed by diffusion and the protective gas stream flowing through the sample tube and this tends to decrease the detector output signal level. These spurious effects obviously interfere with the determination of the true output signal representing the concentration of the element being analyzed for.
Still another disadvantage of prior art devices is the fact that a separate sample is required for each element sought to be determined. In other words, a single sample cannot be subjected to analysis for two different elements. Consequently, it is necessary to subject duplicate samples to drying, ashing and decomposition; and it is also necessary to change the spectral light source after one analysis is completed to substitute the appropriate source for the second element.
With the foregoing state of the art in view, it is the primary general object of the invention to overcome or at least mitigate the problems and shortcomings outlined above.
A more specific overall object of the invention is to provide a novel method and apparatus which materially increases the rate at which sample analysis can be accomplished by means of flameless atomic absorption spectroscopy.
A further object is the provision of an improved method and apparatus for atomization of a sample for atomic absorption spectroscopic analysis which enables determination of more than one element from a single sample.
Another object is to provide a novel method and apparatus for flameless AA sample atomization in which the atomic cloud is present in the analysis beam for a relatively longer time and at a relatively more constant volume and density than in comparable prior art devices.
To the accomplishment of the foregoing and other objectives, the invention contemplates a method of atomization of a sample for atomic absorption spectroscopic analysis in which the sample is subjected sequentially to discrete ambient temperature increments to effect, in distinct, progressive stages, drying, ashing and atomization.
In one of its preferred forms, the apparatus for carrying out the foregoing method may comprise a plurality of chambers disposed in proximity to one another and a conveyor for transporting the sample to be analyzed in sequence to each of such chambers. The chambers are heated to respective predetermined temperatures, the chamber which is lattermost in the sequence being heated to the temperature required to effect atomization of the sample. This lattermost chamber has a radiation transparent passage through which a beam of radiation of selected spectral characteristics is passed to a detector arrangement for determining the degree of absorption of the beam by the atomized sample.